(a) Field of the Invention
The present invention relates to a semiconductor device including at least one static induction thyristor and a photosensitive control element connected to a gate thereof.
(b) Description of the Prior Art
The conventional thyristor which, basically, is formed by a four-layer structure of pnpn has the drawbacks that it is difficult to carry out switching-off action only by a control of gate voltage, and that even when the switching-off is carried out only by this gate voltage, its speed is very low. In contrast thereto, a static induction thyristor (hereinafter to be called SIT) which is basically constructed by a gated diode structure, i.e. an anode region, cathode region and gate means integrated in either the anode or cathode region, has the features that switching-off operation by the gate voltage is easy, and that its switching-off time is short.
Typical structural examples of the conventional SIT are illustrated in FIGS. 1A to 1E.
FIGS. 1A and 1B are sectional views of the SIT having a surface gate structure and FIG. 1C shows a sectional view of the SIT having an embedded gate structure. In the figures, p.sup.+ regions 11 and 14 represent an anode region and a gate region. respectively. An n.sup.+ type region 13 represents a cathode region. An n.sup.- type region 12 represents a first region for constituting a channel. Numerals 11', 13' and 14' represent an anode electrode, a cathode electrode, and a gate electrode, respectively which may be made of a layer of Al, Mo, W, Au or other metals, or a low resistivity polysilicon, or their composite layer structure. Numeral 15 represents an insulating layer made of SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, AlN or like substances, or their mixture, or their composite insulating layer. An n region 16 is of relatively high impurity density, whose thickness is small added to the channel region to restrict the hole injection from the anode.
In FIG. 1D which shows a cross section of the SIT having an insulating gate structure, a p.sup.+ region 21 and an i region 22 are an anode region and a first region consituting a channel, respectively, and n.sup.+ regions 23 and n region 27 are cathode regions and regions for restricting hole injection from an p.sup.+ region 21, respectively. p regions 28 extend vertically with respect to the drawing sheet and reach the surface of the wafer at a suitable position so that they can be connected through electrodes to for example, the cathode region. A reference numeral 25 is the previously mentioned insulating layer. Reference numerals 21', 23' and 24 are the anode electrodes, the cathode electrodes and the insulating gate electrodes, respectively, which were mentioned before.
For example, the p.sup.+ gate region 14 in FIG. 1A to 1C is formed in a striped or mesh pattern in plane view.
The distance between the adjacent p.sup.+ gate regions 14 and the impurity density of, for example, the n.sup.- regions (in the vicinity of the p.sup.+ gate region) are selected such that when a predetermined negative voltage is applied to at least the p.sup.+ gates, the n.sup.- region between the p.sup.+ gate region 14 becomes completely depleted and there is a high potential barrier formed in a front face of the cathode. Further it is designed such that even when the maximum forward blocking voltage is applied, the depletion layer extending from the p.sup.+ gate region 14 does not extended to the anode region so that there is a neutral region of a predetermined thickness left in the n.sup.- region or n region 16 in front of the anode region.
The operation of the SIT thus constructed and the sizes and impurity densities of the respective regions thereof are disclosed in detail in Japanese Patent Application No. 54-8366 of the present applicant.
Since, in the SIT, the switching between conduction and non-conduction is controlled by controlling the potential distribution in the vicinity of the cathode thereof by the gate voltage, it is easy to cut-off d.c. current at a high speed. In the structure shown in FIG. 1A it is possible to design an SIT whose forward blocking voltage is of substantially the same order as that of the reverse breakdown voltage. In the structure shown in FIGS. 1B, 1C or 1D, on the other hand, the same forward blocking voltage as that of the SIT in FIG. 1A can be achieved by using an element whose thickness is substantially half of the thickness of the element in FIG. 1A. Further, the operation speed is high and the forward voltage drop is small which are advantageous. However, the reverse breakdown voltage thereof is small. Therefore, in order to use the SIT having the structure shown in FIG. 1B, 1C or 1D in some device which requires high reverse breakdown voltage, it is usual to connect a Schottky diode or the like in series therewith.
In order to improve the temperature characteristics of the SIT, it may be advisable to employ the opposite conduction type structure. That is, this structure may be provided by substantially regularly and selectively changing the p.sup.+ anode region 11 or 21 to n regions and connecting them through electrodes. One example of such structure is shown in FIG. 1E. In FIG. 1E, the resistance of portions of the n region 16 which are parallel to the anode surface and adjacent to the n.sup.+ regions 20 is selected such that there is substantially no voltage drop when electrons thermally excited in the high resistance 1 region 12 flow thereinto.
FIGS. 2A and 2B show symbol markings of the junction type SIT and the insulating gate type SIT, respectively. As shown, each of these SITs has a diode in the drain side thereof.
FIGS. 3A and 3B show two examples of typical structures conventional photo-sensitive semiconductor elements in which FIG. 3A shows a photo-conductive element and FIG. 3B shows a photo-transistor.
In FIG. 3A, an i region 32 whose resistance is very high and which can be considered an insulator is formed on an n.sup.+ region 31. On both sides thereof, ohmic electrodes 31' and 32' are formed, respectively. The electrodes are, in this example, of In.sub.2 O.sub.2 or SnO.sub.2 which are transparent materials. A lower resistance polysilicon may also be used for them. When the electrode 32' is hardly provided directly, it may be advisable to convert a thin surface portion of the i region 32 into n.sup.+ region and then the transparent electrode 32' may be formed thereon.
When the element is illuminated resulting in electron-hole pairs in the i region 32, an electric current will flow. In FIG. 3A, when the electrode 32' is a Schottky electrode, the element may operate as a Schottky diode which is responsive to incident light L with a voltage application thereto being such that the potential at the Schottky electrode side is lower than that at the ohmic electrode 31'.
In FIG. 3B, which shows the photo-transistor, an n.sup.+ region 44, a p region 43, an n.sup.- region 42 and an n.sup.+ region 41 form an emitter region, a base region, a high resistance layer and a collector region, respectively. The photo-transistor has transparent electrodes 41' and 44' for a collector electrode and an emitter electrode, respectively. The n.sup.+ region 44 and the p region 43 are thin similar to those of the usual bipolar transistor.
Most of the incident light is absorbed by the n.sup.- region 42. When a positive voltage is applied to the collector electrode 41', electrons excited by the light flow into the n.sup.+ collector region and are absorbed therein.
On the contrary, holes flow into the p base region, which is a floating region, and accumulate therein. When the accumulated holes become excessive, the p base region 43 is positively charged and so the potential barrier thereof against electrons in the base region is lowered, resulting in that electrons flow from the emitter region to the base region from which they flow into the collector region. In other words, the photo-transistor becomes conductive with light.
In FIG. 3C which shows a thyristor, an n.sup.+ region 55 and a p.sup.+ region 51 form a cathode region and an anode region, respectively. A transparent electrode 55' is provided on the cathode region 55 and an anode electrode 51' is formed on the anode region 51.
When a positive voltage is applied to the anode electrode 51' and the cathode is illuminated with light L, electrons and holes photo-excited in an i region flow into an n region 52 and a p region 54, respectively. Therefore, the n region 52 is negatively changed and the p region 54 is positively changed. Consequently the barrier potentials against the cathode and anode regions respectively, are lowered and electrons and holes are injected from the cathode and anode regions thereto, respectively, resulting in a conduction state of the tyristor.
Since the thyristor in FIG. 3C has multiplication mechanisms for carrier injection on both sides thereof, the photo-sensitivity thereof is very high.
FIG. 3D shows a p.sup.+ -i-n.sup.+ photo-diode having a transparent electrode 63' and an electrode 61' to which a positive voltage is applied.
In each of the elements in FIGS. 3A to 3D, most of light is absorbed in the i region or n.sup.- region to produce electron-hole pairs therein. Therefore, when the electric field strength in such region is selected as being slightly lower than the avalanche field strength, a large amount of carriers may be produced due to the avalanche multiplication mechanism. Therefore, the sensitivity may be more improved. As will be clear, the avalanche multiplication mechanism will disappear by lowering the voltage between the electrode even if the element is in conductive state. It should be noted that the elements in FIGS. 3A to 3D are mere typical and simplest examples and there are many modifications thereof. The thicknesses of the high resistance regions 32, 42, 53 and 62 in the photo-sensitive devices in FIGS. 3A to 3D should be substantially on the order of the light penetrating depths thereinto in view of the highest efficiency. Another example of the photo-sensitive device may be the photovoltiac device.
The SIT and the photo-sensitive semiconductor elements have been described hereinbefore. Although the SIT is characterized by the large operating voltage and current and high switching speed, it is difficult to apply a single SIT in handling a large electric power such as in d.c. power transmission. The breakdown voltage of a single SIT is determined by the thickness of the n.sup.- region or i region 12 in FIGS. 1A to 1D, because the electric field strength in at least this region must be weaker than the field strength at which the avalanche breakdown commences. This field strength may be around 200 KV/cm for Si. Further, the thickness of the region 12 is required to be equal to or smaller than the diffusion depth of electron and/or holes. However the maximum forward blocking voltage of the SIT is substantially determined by the semiconductor material to be used. For example when Si is used, the value may be around 5,000 to 10,000 volts. Therefore, it is necessary to connect a plurality of SITs each capable of carrying a current of such as 1,000 amperes in the conductive state and having the forward blocking voltage of, for example, 5,000 or 10,000 volts in series with each other to provide a sufficient breakdown voltage and to connect a plurality of the series connected SITs in parallel with each other to provide a sufficient current capacity. In this case, however, it becomes relatively difficult to control the thyristor array between conduction and non-conduction by using an electric signal. Therefore, it is proposed to control it optically.